"Other than the United States, only a few countries — among them India, Thailand, Ukraine and Mexico — allow paid surrogacy. As a result, there is an increasing flow in the opposite direction, with the United States drawing affluent couples from Europe, Asia and Australia. Indeed, many large surrogacy agencies in the United States say international clients — gay, straight, married or single — provide the bulk of their business.

The traffic highlights a divide between the United States and much of the world over fundamental questions about what constitutes a family, who is considered a legal parent, who is eligible for citizenship and whether paid childbirth is a service or exploitation.

In many nations, a situation that splits motherhood between the biological mother and a surrogate carrier is widely believed to be against the child’s best interests. And even more so when three women are involved: the genetic mother, whose egg is used; the mother who carries the baby; and the one who commissioned and will raise the child.

Many countries forbid advertising foreign or domestic surrogacy services and allow only what is known as altruistic surrogacy, in which the woman carrying the baby receives payment only for her expenses. Those countries abhor what they call the commercialization of baby making and view commercial surrogacy as inherently exploitive of poor women, noting that affluent women generally do not rent out their wombs.

But while many states, including New York, ban surrogacy, others, like California, welcome it as a legitimate business. Together, domestic and international couples will have more than 2,000 babies through gestational surrogacy in the United States this year, almost three times as many as a decade ago. Ads galore seek egg donors, would-be parents, would-be surrogates. Many surrogates and intended parents find each other on the Internet and make their arrangements independently, sometimes without a lawyer or a formal contract.

The agencies that match intended parents and surrogates are unregulated, creating a marketplace where vulnerable clients yearning for a baby can be preyed upon by the unscrupulous or incompetent. Some agencies pop up briefly, then disappear. Others have taken money that was supposed to be in escrow for the surrogate, or failed to pay the fees the money was to cover.

Surrogacy began in the United States more than 30 years ago, soon after the first baby was born through in vitro fertilization in England. At the time, most surrogates were also the genetic mothers, becoming pregnant through artificial insemination with the sperm of the intended father. But that changed after the Baby M case in 1986, in which the surrogate, Mary Beth Whitehead, refused to give the baby to the biological father and his wife. In the wake of the spectacle of two families fighting over a baby who belonged to both of them, traditional surrogacy gave way to gestational surrogacy, in which an embryo is created in the laboratory — sometimes using eggs and sperm from the parents, sometimes from donors — and transferred to a surrogate who has no genetic link to the baby.

But thorny questions remain: How much extra will the surrogate be paid for a cesarean section, multiple births — or loss of her uterus? What if the intended parents die during the pregnancy? How long will the surrogate abstain from sex? If she needs bed rest, how much will the intended parents pay to replace her paycheck, and cover child care and housekeeping?

“The gestational carrier has to agree to follow medical advice, but there has to be some level of trust,” said Andrew W. Vorzimer, a Los Angeles surrogacy lawyer who advises on many arrangements that have gone awry. “Once everyone goes home and the doors are closed, there’s no way to really monitor what’s going on.”

Since the Baby M case, the common wisdom has been that the main risk for parents is the surrogate’s changing her mind. But Mr. Vorzimer, who has tracked problem cases in the United States over the years, said it was the reverse: Trouble most often starts with the intended parents. One intended mother decided, well into the pregnancy, that she could not raise a child that was not genetically hers. Another couple, after a divorce, offered the surrogate mother money to have an abortion.

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Critics sometimes draw an analogy to prostitution, another subject that raises debate over whether making money off a woman’s body represents empowerment or exploitation.

In Canada, as in Britain, payment for surrogacy is limited to expenses.

“Just like we don’t pay for blood or semen, we don’t pay for eggs or sperm or babies,” said Abby Lippman, an emeritus professor at McGill University in Montreal who studies reproductive technology. “There’s a very general consensus that paying surrogates would commodify women and their bodies. I think in the United States, it’s so consumer-oriented, so commercially oriented, so caught up in this ‘It’s my right to have a baby’ approach, that people gloss over some big issues.”

Germany flatly prohibits surrogacy, with an Embryo Protection Act that forbids implanting embryos in anyone but the woman who provided the egg. Ingrid Schneider of the University of Hamburg’s Research Center for Biotechnology, Society and the Environment said it is in children’s best interest to know that they have just one mother.

“We regard surrogacy as exploitation of women and their reproductive capacities,” Dr. Schneider said. “In our view, the bonding process between a mother and her child starts earlier than at the moment of giving birth. It is an ongoing process during pregnancy itself, in which an intense relationship is being built between a woman and her child-to-be. These bonds are essential for creating the grounds for a successful parenthood, and in our view, they protect both the mother and the child.”

With all that is known about adopted children’s seeking out their biological parents, other European experts say, it is wrongheaded to create children whose relationship with the woman who provided the egg or carried them will be severed.

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The restrictions in many countries have been a boost for American surrogacy. For overseas couples, the big draw is the knowledge that many states have sophisticated fertility clinics, experienced lawyers, a large pool of egg donors and surrogates, and, especially, established legal precedent.

“We chose the United States because of the certainty of the legal process,” said Paulo, an engineer and scrub nurse. “Surrogacy is very secretive in Portugal. People don’t talk about surrogacy, and it’s hard to get any information. In the United States it is all clear.”

But it is not cheap. International would-be parents often pay $150,000 or more, an amount that rises rapidly for those who do not get a viable pregnancy on their first try. Prices vary by region, but surrogates usually receive $20,000 to $30,000, egg donors $5,000 to $10,000 (more for the Ivy League student-athlete, or model), the fertility clinic and doctor $30,000, the surrogacy agency $20,000 and the lawyers $10,000. In addition, the intended parents pay for insurance, fertility medication, and incidentals like the surrogate’s travel and maternity clothes.

Because surrogacy is so expensive in the United States, many couples travel to India, Thailand or Mexico, where the total process costs half or less. But complications have arisen — as in the case of a couple stuck in India for six years, trying to take home a baby boy, whom genetic testing had found not to be related to them, apparently because of a mix-up with the sperm donation.

Four years ago, according to Stuart Bell, the chief executive of Growing Generations, a Los Angeles surrogacy agency, only about 20 percent of its clients came from overseas, but now international clients are more than half. Other agencies report the same trend.

“Anyone who can afford it chooses the United States,” said Lesa A. Slaughter, a fertility lawyer in Los Angeles."